Solid insulation for electrical apparatus

ABSTRACT

An improved thermally stable polymeric insulation for use in electrical apparatus and particularly in transformers operated in the presence of transformer oil is a substitute for the conventional cellulosic pressboard, which improved insulation consists of an isotactic polymeric hydrocarbon resin in at least partially crystalline form and cross-linked, 1,2-butadienes and copolymers having a low dielectric constant substantially matching that of the liquid dielectric and having relatively low swelling characteristics when immersed in hot liquid petroleum oil dielectric over an extended period of time.

y g relarsed in hot liquid petroleum oil dielectric over an extended period of time.

y in transrmers operated in the presence of transformer oil is a substitute for the conventional cellulosic pressboard, which improved insulation consists of an is ymeric hydrocarbon resin in at least 3,775,719 [451 Nov. 27, 1973 Birks, London Heywood & Company Ltd., 1960, pages 140 and 241 relied upon.

otactic -polpartially crystalenes and copolyant substantiall 3,548,354 12/1970 3,078,139 2/1963 Brown et'al. OTHER PUBLICATIONS Monroeville, both Modern Dielectric Materials,

Primary Examiner-Thomas J. Kozma Att0rney-F. Shapoe [57] ABSTRACT An improved thermally stable polymeric insulation for use in electrical apparatus and particularl fo H0lf 27/12 3 36/58; 94 60 206; line form and cross-linked, l,2-butadi 317/258; l 74/25'R, 25 026 R, 17 LF, 110 mers having a low dielectric const 1 10 SY, 160 13 matching that of the liquid dielectric and havin tively-low swelling characteristic's when imme 336/58 15 Claims, 6 Drawing Figures SOLID INSULATION FOR, ELECTRICAL APPARATUS I r Inventors: Gordon C. Gainer, Pittsburgh;

Russell M. Luck, of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Apr. 14, 1972 Appl. No.: 244,183

v Related Application Data [63] Continuation-impart of Ser. No. 146,238, May 24,

U.S. 336/58, 174/25 R, l74/l7 LF, 174]] 10 SR, 174/1 10 S Y, 336/60, 336/94, 336/206 Int. Cl. Field of Search R, 110 SR References Cited v UNITED STATES PATENTS l0/l97l United States Patent Gainer et-al.

[73] Assignee:

[22] Filed:

PATENTEUNUV27 ms sum 1 or 3 FIG.2

I I I i I I i n I I I i i i I I PATENTEBNUVZY ma SHEET 0F 3 (ISOTACTI C POLYSTYRE NE VAT/who POLYSTY'RENE 20% t- BUTYLSTYRENE- 1,2- POLYBUTADI ENE COPOLY MER 2O /0 STYRENE-l ,2- POLYBUTADIENE (JOPOLYM ER ANNEALED ISOTACTIC POLYSTYRENE CROSS-LINKED LZ-POLYBUTADItNE hE'l lN TIME (DAYS) 7 FIG?) L/ATAC TIC POLYSTYREN E p0 UNANNEALED ISOTACTIC POLYSTYRENE FIG.4

ANNEALED ISOTACTIC POLYSTYRENE CROSS-LINKED |,2 POLYBUTADIENE TI ME( DAYS) PATENTEBNUVZHQH sum 3 0r 3 O ZHOUR ANNEAL l7 5C o 6HOUR ANNE/\p a 175 c 0 4HOUR ANNE/v |?s"c FIG5 230 kiwi;

TI ME (HOURS) CI BISPHENOL EPOXY m 'I,ZPOLYBUTADIENE(LOW MOLWT.)

o |,2POLYBUTADIENE(HIGH MOLWT.)

zoammmmzou FIGL6 TIME (DAYS) BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relatesto an improved solid polymeric insulation material for use in an oil immersed transformer and, more particularly, it pertains to insulation material especiallysuitable for use ineither a core or shell-form power transformer.

2. Description of the Prior Art Mineral oil impregnated cellulosic materials such as paper, cotton cloth, cotton tape,pressboard, and wood have long been employed in-th'e electrical industry as insulation for various types of apparatus, because of their excellent overall dielectric properties, satisfactory initial dielectric strength and low cost. The electrical and physical properties of cellulosic material, however,

deteriorate at an ihereasing'rate when the operating temperatures of the electrical. apparatus rise above about 100C whether exposed to air or in contact with fluid dielectric compositions such transformer oil. The deterioration in the physical properties is accompanied by a correspondingdecrease in electrical insulation properties. For these reasons, there has been a need for more satisfactory solid spacing materials for use as insulators for the ele ctric'al windings in an oil immersed transformer particularly wher'ethe highest electrical stresses may be present and good impulse strength is required.

The primary requirements for sucha solid insulator are a low dielectric constant and low swelling characteristics when immersed intransfor mer oil at elevated temperatures .over an extended period of time. With regard tothe dielectric constant, in oil immersed transformers a reduced intensity of electric stress may be obtained if properlyqselectedsolid dielectrics are uti-' lized as substitutes for oil, im regnatedeellulose whose dielectric constant is 3.75. For best results, such solid insulator should have a dielectric constant of about 2.2 that more nearly matches the dielectricconstant of the transformer oil. The dielectric constant of the refined petroleum oil used in'transforrners'is about 2.2. Generally, a solid insulating material of low dielectric constant closely or exactly matching that of oil as a substit'u'te for oil impregnated pressboard in areas of high electric stress (including particularly high electric stress in the case of impulse currents due to lightning strokes), enables a reduction in insulating thickness or spacing to as much as about 2/3 of that now used.

One of the principal problems associated with achievement of that reduction of insulation clearance that available low dielectric constant "resins,'such,as

hydrocarbon polymers, are excessively swollen by contact with hot transformeroil so that after a briefperiod of time it renders the resins unserviceable. Many commercially available resins, such as ata'c'tic polystyrene,

polyethylene, and isotactic polypropylene, soften "excessively at l25C and show considerable swelling weight pickup.

SUMMARY OF THE INVENTION In accordancewith this invention, it has been found that the'foregoing problems may be overcome by providing-an improved solid polymeric insulation for use in transformers, reactors and other electrical apparatus in which petroleum oil is used as insulation, which polymeric insulation consists of a material selected from at leasti in'e of the groursconsisting of certain thermosetting 1,2-polybutadiehe hydrocarbon resins and certain copolymers of these, and partially crystalline isotactic polystyrene, which materials have a dielectric constant of from about 2.0 to about 2.7 and which-closely corresponds to that of the oil, and have long term swelling properties of less than about 15 weight per cent in petroleum oil at a temperature of about 125C.

The advantages arising fromthe use in oil impreghated electric apparatus of such solid insulating materials instead of oil ir'npregnated cellulosic pressboard, are cost savings, due to smaller size and total. lower weight by virtue of reduced insulationclearance otherwise required, and longer life of transformers and other oil insula'ted apparatus.

Thus, a cellulosic pressboard cylinder of a thickness of 1 inch, is replaced by a cylinder of isotactic polystyrelic of a thickness of about 0.65 inch with equally efv 'fective electrical insulation.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference is made to the attached drawings in which similar numerals refer to similar parts throughout the several views of the drawings, and in which:

FIG. 1 is a schematic elevational view of a magnetic core and winding assembly;-

FIG. 2 is afragmentary enlarged, perspective view, partly in section, in detail of a core type transformer;

FIG. 3 is a graph showing the weight per cent gain for various types of resins versus time;

FIG. 4 is a graph showing the volume percent swell for various types of resins versus time;

FIG. 5 is a graph showing the weightpickup of transformer oil in isotactic polystyrene as a function of time of anneal; and v FIG. 6 is a graph showing the per cent compression under a pressure of 750 psi, load in -oil at- C for various resins versus time.

DESCRIPTION OF THE PREFERRED I EMBODIMENT A core type transformer is specifically disclosed and described hereinbelow, but other forms of transformers such as the shell' 'type, reactors and other oil immersed electrical apparatus are suitablefor the practice of this invention.

In FIG. 1, there is shown schematically a threephase core type'of power transformer generally indicated at 10, comprising a casing 11 'in' which is disposed a triagnetic core 12 having three spaced legs 14 with a winding assembly 16 which comprises primary and secondary coils, disposed around each leg. Eachwinding assembly 16 is comprised of separate coil sections 18, each of which includes one or more pancake coils or coil discs. Radial spacers 20 separate each coil section tions 18. The type and placement of the interconnections 22 between the discs depends'upon the type and class of the winding assembly 16, for which reason the interconnections 22 as shown areillustrative only. Terminals 24 and 26 prvide means to electrically connect the winding assembly 16 to the associated apparatus or circuitry, however, other terminals at other terminal positions may be utilized.

One practical construction of a core type transformer is more particularly illustrated in FIG. 2, wherein the leg 14 of the core 12 and the winding assembly 16 are shown partly in section with their associated solid insulation. The core 12 is provided on each side with lower channel members 40 fastened thereto for supporting the working components of the, transformer in casing '11, and upper channel members42 between which channel members the. windings 16 are clamped. To accomplish this, a heavy steel ring 44 rests on the lower channel members 40, while an upper steel ring 45 abuts against the bottom of the upper channel members 42. A flanged ring 46 of solid insulation of molded fiber reinforced resinous material rests on the ring 44 and a series of spaced insulating members in ring arrangements 47 and 49 are disposed on the flanged ring 46. Inner phase insulation barriers of solid insulation are provided to isolate each ofv the transformer phases from each other. The barriers comprise a rectangular flanged bottom sheet 48 with its center cut out just beyond ring 49 on which it is disposed, insulating side walls 50 bolted to the bottom sheet'48'by bolts 52 of electrically insulating material such as fiber reinforced molded resin, and a flanged top sheet 53 which is a mirror image of bottom sheet 48, also bolted by bolts 52 v to the side walls 50, the whole forming a rectangular compartment. The transformer casing walls usually close the open front and rear sides of the compartment. In the larger transformers the legs 14 are not rectangular, but are composed of laminations'that diminish stepwise in width from the widest extending from face 56 to a corresponding face on the left side in FIG. 2, to the narrowest width at face 55. The stepwise arrangement of the larninations results in corners 7 in which are placed filler rods or strips 58. Thefiller strips 58 may be round or quarter round or of other suitable shape and are of a solid insulating material. The purpose of the filler strips 58 is to support a cylindrical shell 60 of solid insulation which shell in turn supports and electrically insulates low voltage windings 62 of the electrical coil comprising a series of pancake coils 1'8.

. The pancake coils 18 are separated by two types of in-' sulating radial spacers 20, which may comprise relatively thick spacers 63 and thinner spacers 65 to enable flow of insulating oil between the coils. The thick spacers 63 enable a greater flow of cooling oil as well as providing a space to apply electrical taps, and for other purposes.

After the entire low voltage winding is built up by stacking coils 18 on ring 47 withv intervening insulating radial spacers 63 and 65 between them, a cylindrical insulating shell 64 closely conforming to the other periphe ry of winding 62 is slipped over it, then a larger outer tubular insulating shell 66 is placed about the shell 64 and vertical spacer strips 68 are introduced in a predetermined pattern in thespace between shells 64 and 68. The vertical spacer strips fit tightly into the space so as to enable the shells 64 and 66 tostrengthen each other, while enabling oil to flow freely upwardly 4 between the shells when th'e transformer is in operation.

The high voltage winding 70 is then applied by slipping individual pancake coils over the shell 66,at least some of the coils fit closely, the bottom coil resting on the insulating sheet 48 and concentric with ring arrangement 49. Thick radial spacers 72 and thin radial spacers 74 are disposed between each pancake coil of winding 70. 7

In practice, a thin insulating tubeis applied to the outside of the winding 70 to direct the flow of oil. Schwab US. Pat. No. 3,548,354 :shows various arrangements of such insulating tubes and baffles or barriers about coils to direct oil flow over the coils.

After both the high voltage windings -70 and low voltage windings 62, as well as the several insulating shells 60, 64 and 68, are placed around core leg 14,- an upper spaced series of insulating members in a ring arrange-' ment 76 is placed over the top pancake coilof the low voltage winding 62, the upper rectangular insulating sheet 53 is placed over the top pancake coil of the high voltage winding 70, and asecond spaced series of insulating members in a ring arrangement 77 is disposed on sheet 53 in line with winding 70, a flanged solid insulating ring 80 and the steel ring 45 is then disposed on top of the windings. The rest of the'magnetic core comprising the horizontal yoke 82 is then assembled on the vertical legs 14.The upper channel members 42 are put in place and suitable bolts are applied to bolt the channels to the magnetic core and to apply pressure to the plates 45-44 so as to compress the windings 62-70 therebetween. The electrical connections to the several coils at terminals 24 and 26 and any coil taps have also been made during the assembly process. The transformer tank 11 is filled with oil so as to cover thecore l2 and windings 62 and 70. r 7 i When the transformer is put into operation, hot oil flows over the insulating tubes 60, 64,66, the strips 58 and 68, and also the various insulating members 46, 47, 48, 53, 63, 65, 72, 74, 76, 77 and 80. All of these insulating tubes, strips and members are also subject to heavy pressures both static, as when the-transformer coil assembly is bolted firrnly .together,- and dynamic when surges and. short circuits p'assthrough thewindings and affect the transformer. The sudden increase in current flow during surges and short circuits on the windings creates shock forces that severely stress the spacers and tubes. It is necessary that the hot oil not react with or adversely affect the resinous insulation comprising these tubes, strips and spacers.

Furthermore, the insulating members immersed in the insulating oil must be capable of withstanding the voltage and electric stresses applied to the windings.

When the solid insulating materials have a dielectric constant nearly the same as that of the oil, namely from 2.0 to 2.7 and preferably about 2.2, the electrical voltage stress is graded more uniformly and neither the oil nor the solid electrical insulation is subjected to a disproportionate voltage gradient. Consequently, the solid insulating materials, namely, tubular-members, strips, spacers and the like resinous insulating materials as shown in FIG. 2, may be much t-hinner than more conventional solid insulating materials with dielectric 'con-' stants of 3.7 and more.

Petroleum base dielectric liquids such as those commonly known as transformer oil are highly refined petroleum oils of a very low acid number. Stabilizers and ent therein.

antioxidants, such as para-t-butylphenol, may be pres- In accordance with this invention, the several solid resinous electrically insulating parts subject to the high electrical voltages and stresses, particularly comprising members 46, 47, 48, 76, 77 and 80, the radial spacers 63, 65, 72 and 74, the vertical spacers or strips 58 and 68, and the insulating tubular cylinders 60, 64 and 66 are composed of certain hydrocarbompjolymers having dielectric constants substantially matching that of the transformer oil and resistant to reactionwith or adverse swelling action of the oil at'temperatures of up to about 125 C for extended periods of time.

More specifically, the hydrocarbon polymers found to fulfill the requirements of low dielectric constant and minimal swelling characteristicsin petroleum oil consist of a material selected from the group consisting of thermosetting cross-linked l,2-polybutadiene hydrocarbon resins and copolymers thereof with vinyl monomers, and at least partially crystalline isotactic polystyrene.

- The thermosetting cross-linked 1,2-p6lybutadiene hydrocarbon resins and certain vinyl copolymers thereof which arederived from butadiene. exhibit low dielectric constants and low swelling characteristics in hottransformer oil. I

6 rate is relatively slowc'ompared with other crystallizable polymers, such as polyethylene or polypropylene. Amorphous isotactic polystyrene has properties somewhat similar to those of conventional atactic polystyrene.

However, crystalline isotactic polystyrene has a high melting temperature showing a first order transition temperature of about 240C. It is insoluble in most common solventsfo'r polystyrene, and, as a result of the spherulitic structure ofthe crystalline phase, is opaque. From X-ray data the density of the 100 percent crystallinep'olymer is calculated to be 1. 12.Polystyrene with a high degree of isotacticity is readily prepared. On annealing, partially crystalline isotactic polystyrene has been obtained, generally with less than percent relative crystallinity.

As indicated by Sorenson and Campbell in Preparative Methods of Polymer Chemistry, the various polymeric configurations of polystyrene may be visualized by imagining the carbon-carbon polymer chain laid out on a plane in the extended zigzag conformation. If'the substituents (in this case, the phenyl group) from the monosubstituted vinyl monomer are arranged at random above and below the plane of the carbon chain, the polymer lacks stereochemical regularity and is ca lled atactic. The chain configuration is as follows:

RR -'HH- RH H x wherein represents the phenyl group.

Generally, the titanium-based catalysts give, by a 3 5 mechanism which is still unclear, a stereoregular chain H 40 chain structure:

in which each succeeding asymmetric carbon hast-he same configuration as the preceding one.

If the substituents all fall on one side of the plane,'t he polymer is said to be isotactic; and has the following lsotactic polystyrene may. be-produced by polymer-- ization of styrene with stereo-specific catalysts of the Finally, if the substituents fall alternately above and below the plane of the chain; the polymer is designated- Ziegler-Natta type as set forth more particularly by syndiotactic; whose chain configtirationis as follgws science Publishers, Inc., NY. 1959, and (3) Wayne 80- 0 *i'nson and Tod W. Campbell Preparative Methods of 'Polymer Chemistry, l96l,pages 201 and 202, Inters'cience Publishers, Inc. As a result of the regular isotactic structure it can be crystallized and has a threefold helix-chain coi'ifoimation. lsotactic polymer can' exist in the amorphous or crystalline state. Samples quenched from the' inelt are amorphous but become crystalline if annealed for some time at a temperature below the crystalline melting point. Thecrystalliaation R, an

R H R a I The stereoregularity permits the chains to crystallize, hence the properties of the isotactic and syndiotactic polymers differ markedly from the random counterpart. Thus, atactic polystyrene is clear, noncrystalline, and of a low melting temperature, while stereoregularu polystyrene is hazy like nylon, crystalline, orientable and of a high melting temperature. The nature of the R group also affects the melting point markedly; in general, the bulkier it is the higher meltingthe polymer.

Theoretically, the syndiotactic polymer should crys-' tallize in a sufficient amount that it is of potential interest in this invention.

Unlike ordinary atactic polystyrene, the crystalline isotactic polystyrene is relatively insoluble in such sol vents as benzene, chloroform,acetone, and more particularly it exhibits low weight increase when contacted with hot mineral oil over a period of time. Isotactic polystyrene has a low dielectric constant, high melting point, and a low power factor. It is a solid insulating material that in highly crystalline form exhibits low swelling in hot transformer oil. Moreover, the material has a relatively low cost because it is readilymade from styrene by polymerization with a special catalyst. Because of such low cost the isotactic polystyrene can be used in transformers as a highly improved substitute for paper, pressboard, wood and similar cellulosic insulation.

The partially cross-linked 1,2-polybutadiene hydrocarbon resins and copolymers thereof with vinyl monomers and the partially crystalline isotactic polystyrene can be employed alone or with various fibrous fillers to produce strong molded members. It is important that the fibrous fillers be of a dielectric constant close to that of the resin. A particularly suitable fibrous material is crystalline isotactic polystyrene made into fibers, as set forth in U. S. Pat. No. 3,078,139. The fibers can be woven into cloth, matted, chopped or employed as strands and impregnated with partially crystalline isotactic polystyrene, or the 1,2-polybutadiene hydrocar-, bon resin or the latter resin partly dissolved in vinyl monomers such 1 as monostyrene. The fibrous-resin composite can be laminated or molded under heat and pressure into plates, tubes, sheets, rods, washers and other shapes for use in transformers or other-electrical apparatus. Polypropylene fibers have excellent strength and a satisfactory dielectric constant, but swell excessively in hot oil. Short'chopped polypropylene fibers completely enclosed or embedded in the resin, such as partially crystalline isotactic polystyrene can be em-' ployed as long as no fiber ends extend to or protrude from the surface of any member.

The partially crystalline isotactic polystyrene may by I admixed with small amounts, up to several percent by weight, of other polymers prior to crystallizing the isotactic polystyrene. The added polymer may be also crystallizable. After crystallization, the mixed polymer product may be shaped to the desired insulator member and used in the practice of the invention. In some cases, there may be added to the monostyrene small amounts of other copolymerizable monomers, as for example, methyl styrene, and then the mixture is poly merized into a copolymer comprising primarily isotactic polystyrene groups and then crystallized. Further, after the isotactic polystyrene is produced, but prior to crystallization, coreactive monomers may be added and reacted in situ to function as end-blocking groups, or the added monomer may be primarily polymerized into polymeric groups relatively homogeneously admixed in the main body of polystyrene, and then the copolymer, or mixture, is crystallized into a body suitable for practicing the invention. The added polymers preferably should be relatively insoluble in oil and have a low dielectric constant comparable to that of the isotactic polystyrene.

- In a similar manner the thermosettable 1,2- polybutadiene hydrocarbon resins and vinyl copolymers thereof may be admixed with small amounts of one or more other relatively, oil insoluble resins. A homogeneously, admixed resinous body may be prepared which is suitable for the practice of the present invention.

It should be understood that the resinous insulating members, such astubes'or'cylinders, separators, rods and spacers, will comprise essentially the crystalline isotactic polystyrene or the thermoset 1,2- polybutadiene hydrocarbon resins and vinyl copolymers thereof, and any added or admixed resin and/or fillers therein will not'appreciably impair the oil insolubility and non-swelling properties thereof, nor raise the dielectric constant ofthe entire insulating member above about 2.7, so that when incorporated in oil filled apparatus, the dielectric stress is not disproportionately distributed as between the oil and this solid insulation.

The following example is exemplary of the invention:

EXAMPLE Sample discs of isotactic polystyrene, atactic polystyrene, and a polymer of 1,2-butadiene with 20 percent by weight of monostyrene were molded at temperatures of from 225 to 250C in a compression 'mold. The isotactic polystyrene discs were then annealed at 175C for about 1 hour so that crystallization occurred, and the samples changed from transparent to opaque.

The discs were then immersed in hot transformer oil and held at C for 60 days and weight gain and volume swell for the isotactic polystyrene as well as the other polymers was found as shown in FIGS. 3 and 4, respectively. In FIG. 3, the weight gain comparisons indicate that atactic polystyrene (curve A) increases rapidly..The isotactic polystyrene (unannealed curve B) has a decrease in weight gain with the passage of time; however, when annealed at 175C for 1 hour to effect a high degree of crystallinity (curve C) it has a greatly reduced weight gain. v

In FIG. 4, somewhat similar results were obtained for the property of per cent volumeswell. The atactic polystyrene (curve D) has'a'very rapid increase in swelling as compared with isotactic polystyrenes, Further, swelling for highly crystalline isotactic polystyrene (annealed curve E), is considerably less than .the noncrystalline isotactic polystyrene in the unannealed state (curve F). More particularly, the as-molded isotactic polystyrene (unannealed curve F) in the relatively noncrystalline state, swells (FIG. 4) up to over 16 percent in volume in 58 days. The swelling, however, was greatly reduced by annealing theisotactic polystyrene to the crystalline state to just over 4 percent in 8 days (curve E). I r

The effect of annealing time on the isotactic polystyrene on the weight pickup is shown in FIG. 5. Samples of isotactic polystyrene were annealed at 175C for periods of 2, 4 and 6 hours and then immersed in transformer oil at 125C for a period of time. The results, shown in FIG. 5, indicate that annealing for the isotactic polystyrene some time between 2 and 6 hours, since an optimum value is'achieved in 4 hours, for a minimumweight pickup in transformer oil.

. As is also shown in FIGS. 3 and 4, the cross-linked l,Z-poIybutadiene-styrene'copblymer resin has a satisfactory property of low swelling and per cent weight gain as compared with the other resins included in this Example. The test samples which provided the data for the curves plotted in FIGS. 3-and 4, were cross-linked by incorporating from' 1.- percent to 2 percent of 'dicumyl peroxide as catalyst inthe viscous resin 'and curing in an oven for 1 hour at] 10C followed by an overnight cure at C. Other cure schedules may be employed dependin "Pe the a iq PQJLZLIEQMEEE FPFL? 2 9 r nonostyr eneand the re sin catalysts used. The samples were then tested for weight increase in hot transformer oil at 125C and displayed an extremely. low volume swell and. low weight gain. The cross-linked 1,2- polybutadiene resins exhibited only 0.7 to 0.8 percent weight increase in 60 days with no visible evidence of change in appearance of theaged samples. (Curve G,

and Curve P1, IG: 4% T is pe, 9f .qrqssrlinlss 1,2-polybutadiene resin is a satisfactory material for insulators in oil filled transformers.

' Electricalmeasurements were'alsomade on the samples of the cross-linked 1,2-polybutadiene resins and the data obtained thereby is listed in the following Table:

TABLE The dielectric constant of 2.3 for cross-linked 1,2 polybutadiene resin is substantially identical to that of between 2.1 to 2.2 for transformer oil. The power losses at 25C are excellent but the losses (3.2 percent) at 60 cycles at 100C were higher than expected or desired and were attributed to the presence of residual catalysts remaining in the polymer. To prove this a sample of no'n-cross-linked l,2-polybutadiene resin in liquid form was extracted in deionized water for several days while the conductivity of the water solution was monitored. Washing was continued until the conductivity of freshly-added deionized water remained low. It was then cross-linked in the manner mentioned earlier and the electrical losses were substantially reduced.

Other sampleswere prepared which consisted of copolymers of 1,2-polybutadiene' with varying amounts of t-butylstyrene and monostyrene. The swelling effect of hot transformer oil on such samples is shown in FIG. 3, where the l,2-polybutadiene content is 80 percent, the balance being styrene (curve I) and t-butylstyrene (curve .1, FIG. 3). Use of styrene or t-butylstyrene as a' possesses excellent properties for per cent of volume' swell and per cent of weight gain it was relatively brittle. By incorporatingin the resin a low dielectric constant reinforcing fiber such as isotactic polypropylene fiber and mat structures, prior to curing in the oven, a better thermoset polymer product was obtained. To enhance the product, the polypropylene fibers were chemically treated to condition and oxidize the surface of the fibers by immersion in sensitizing baths followed by subsequent oxidation in chromic acid-sulfuric acid bath solutions, whereby polar and more readily wetted surfaces were obtained. Polypropylene fibers were particularly suitable reinforcing fibers because of their low dielectric constant so that the overall dielectric constant of the impregnated resin system was not significantly increased. Thus, 50 grams of polypropylene l0 resin containing 2g. of dicumyl peroxide catalyst and cured in a'press as outlined earlier. Such compositions were useful for oil-filled transformer-applications because of their low dielectric constant, low electric loss, and otherwise. improved physical properties. Data on oil swell areshown in FIG. 3, (Curve K).

A polyesterfiber reinforced sample of cross-linked 1,2-polybutadiene resin system was prepared by .im-

pregnating 10 grams choppedpolyester fibers (derived from polyethylene terephthalate) in a liquid resin com prising grams of 1,2-polybutadiene resin, 30 grams of divinylbenzene, with 3 grams of dicumyl" peroxide, and molding to cure in the manner described earlier. Swell data obtained in hot transformer oil are also shown in FIG. 3, (Curve L).

In addition to the foregoing tests, samples of selected, cross-linked 1,2-polybutadiene resins some having high molecular weight and others lower molecular weight were'subjected to compression tests under 750 psi loads in oil at 125C and the results compared with tests on other materials including isotacticpolystyrene, bis-phenol epoxy resin hardened with hexahydrophthalic anhydride and the same epoxy resin filled with 30 percent of cross-linked 1,2-polybutadiene powder. The results as shown in FIG. 6 indicate that under the compression load tests, the high molecular weight crosslinked 1,2-polybutadiene resin had the least physical deformation (compression under load). Next in value was the low molecular weight version. The isotactic polystyrene is found to display nearly identical values 'of compression to filled bisphenol-epoxy resin. The primary disadvantage of cross-linked l,2-polybutadie ne resins in the non-reinforced condition is that the material is fragile and requires care during handling and installation in a transformer.

Other isotactic hydrocarbon substitutedpolystyrene polymers that may be employed are-the isotactic polymers derived from vinyl toluene (i.e., the isomeric methyl styrenes) and from t-butyl styrene in atleast partially crystalline form for example, between 10 to 35 percent crystalline structure.

In summary, certain selected polymeric hydrocarbon resins are shown to have low dielectric constants which closely match that of transformer oil. These selected polymers likewise have relatively low swelling properties when immersed in hot transformer oil over extended periods of time. These properties of low dielectric constant and low swelling are in contrast with certain other hydrocarbon polymers, e.g., polyethylene, isotactic polypropylene, atactic polystyrene, and the like, which have very high swelling, substantially greater than 15 percent by weight, in a few days in the same hot oil medium. As a result, the isotactic polystyrene and selected cross-linked 1 ,2-polybutadiene resins are outstanding materials for use as solidelectrical in fiber. ere .insqmor tsd. in 8- of 1. -PQ ybqtas i 2ns? Rattan?" q fihqsliqt el afifim thssl strisal. in

.siiiatibii fia'viagxaiaaata eaa's'iani ambit. about iif)" to 2.7 and being relatively insoluble in the oil and of low swelling in hot oil, the solid electrical insulation comprising essentially a solid insulating material selected from at least one of the group consisting of thermosetting cross-linked 1,2-polybutadiene hydrocarbon resins and copolymers thereof, and at least partially. crystalline isotactic polystyrene. I g g I 2. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially a thermosetting cross-linked 1,2-polybutadiene hydrocarbo resin.

3. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially at least partially crystalline isotactic polystyrene.

4. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially a ther- 11. The insulation of claim 1, wherein fibers of isotactic polystyrene are imbedded in the cross-linked 1,2-

moset copolymer of 1,2-polybutadiene hydrocarbon resin and a vinyl monomer.

5. The apparatus of claim 3 wherein the polystyrene has a crystalline structure of at least about 10 percent of the total resin.

6. The electrical apparatus of claim 1 wherein the solid material has a dielectric constant of from about 2.0 to 2.5.

7. The electrical apparatus of claim 5 wherein the solid material has a dielectric constant of about 2.2.

8. The electrical apparatus of claim 1 wherein the solid material exhibits a swelling of less than percent by weight over a prolonged period of time when immersed in petroleum oil at a temperature of about 125C.

9. The insulation of claim 1 wherein the solid materialhas a dielectric constant that is substantiallythat of the petroleum transformer oil.

10. The insulation of claim 1, wherein the solid insulating material includes fibrous material having a dipolybutadiene resin.

12. The'insulation of claim 1, wherein fibers of polyester resin are imbedded in the crosslinked 1,2- polybutadiene resin. 7

13. In a transformer comprising a magnetic 'coreand electrical windings disposed on the legs of the core, and I an insulating oil having a dielectric constant of about 2.2 applied to the electrical windings, solid electrical insulation disposed between the electrical windings and the magnetic core, and between portions of the 'electrical windings, the solid electrical insulation being selected from at least one polymeric material from the group consisting of thermoset 1,2-polybutadiene hydrocarbon resin and copolymers thereof with vinyl monomers, and at least partially crystalline isotactic polystyrene, the polymeric material having a dielectric constantgf from about 2.0 to 2.5, and being resistant to swelling when immersed in the insulating oil at temperatures of up to 125C, whereby improved'dielectric strength is exhibited by the combined insulating oil and solid polymeric electrical insulation, and the solid poly- .meric insulation exhibits good mechanical strength:

14. The transformer of claim 13, wherein the solid electrical insulation comprises essentially the polymeric material and a fibrous material combined therewith, to provide a strong insulating material capable of 

2. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially a thermosetting cross-linked 1,2-polybutadiene hydrocarbon resin.
 3. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially at least partially crystalline isotactic polystyrene.
 4. The electrical apparatus of claim 1 wherein the solid insulating material comprises essentially a thermoset copolymer of 1,2-polybutadiene hydrocarbon resin and a vinyl monomer.
 5. The apparatus of claim 3 wherein the polystyrene has a crystalline structure of at least about 10 percent of the total resin.
 6. The electrical apparaTus of claim 1 wherein the solid material has a dielectric constant of from about 2.0 to 2.5.
 7. The electrical apparatus of claim 5 wherein the solid material has a dielectric constant of about 2.2.
 8. The electrical apparatus of claim 1 wherein the solid material exhibits a swelling of less than 15 percent by weight over a prolonged period of time when immersed in petroleum oil at a temperature of about 125*C.
 9. The insulation of claim 1 wherein the solid material has a dielectric constant that is substantially that of the petroleum transformer oil.
 10. The insulation of claim 1, wherein the solid insulating material includes fibrous material having a dielectric constant close to that of the resin and the oil.
 11. The insulation of claim 1, wherein fibers of isotactic polystyrene are imbedded in the cross-linked 1,2-polybutadiene resin.
 12. The insulation of claim 1, wherein fibers of polyester resin are imbedded in the cross-linked 1,2-polybutadiene resin.
 13. In a transformer comprising a magnetic core and electrical windings disposed on the legs of the core, and an insulating oil having a dielectric constant of about 2.2 applied to the electrical windings, solid electrical insulation disposed between the electrical windings and the magnetic core, and between portions of the electrical windings, the solid electrical insulation being selected from at least one polymeric material from the group consisting of thermoset 1,2-polybutadiene hydrocarbon resin and copolymers thereof with vinyl monomers, and at least partially crystalline isotactic polystyrene, the polymeric material having a dielectric constant of from about 2.0 to 2.5, and being resistant to swelling when immersed in the insulating oil at temperatures of up to 125*C, whereby improved dielectric strength is exhibited by the combined insulating oil and solid polymeric electrical insulation, and the solid polymeric insulation exhibits good mechanical strength.
 14. The transformer of claim 13, wherein the solid electrical insulation comprises essentially the polymeric material and a fibrous material combined therewith, to provide a strong insulating material capable of withstanding the loads present during service.
 15. The transformer of claim 13, wherein the solid electrical insulation is employed for tubular insulating members interposed between the electrical windings and between the magnetic core and the electrical windings. 